Solid electrolytes have recently been used more often than (aqueous) electrolyte solutions. This tendency is firstly because those solid electrolytes have good processability in application in electric and electronic materials, and secondly because of the transitions to overall size and weight reduction and electric power saving.
Inorganic and organic proton conductive materials are known.
As the inorganic materials, hydrates such as uranyl phosphate are used. However, it is difficult that the inorganic materials are enough contacted with substrate or electrode interface. As a result, many problems in forming a conductive layer on a substrate or an electrode are caused.
On the other hand, the organic materials include polymers that belong to cation exchange resins, with examples including sulfonated vinyl polymers such as polystyrenesulfonic acid; perfluoroalkylsulfonic acid polymers and perfluoroalkylcarboxylic acid polymers represented by Nafion® (manufactured by DuPont); and polymers obtained by introducing sulfonic acid groups or phosphoric acid groups in heat resistant polymers such as polybenzimidazole and polyether ether ketone.
In the manufacturing of fuel cells, an electrolyte membrane of the perfluoroalkylsulfonic acid polymer is sandwiched between electrodes and heat processed by hot pressing or the like to give a membrane-electrode assembly. The fluorine-containing electrolyte membranes are thermally deformed at relatively low temperatures around 80° C. and can be assembled easily. However, the temperature can rise to 80° C. or above by reaction heat during operation of the fuel cells. In this case, the electrolyte membrane is softened and creeps to cause short circuits between the electrodes, resulting in power generation failure.
To prevent these problems, the thickness of the electrolyte membranes is increased to a certain level or fuel cells are designed such that the power generation temperature will not exceed 80° C. Consequently, the maximum output of power generation is limited.
To solve the problems with low thermal deformation temperature and poor mechanical characteristics at high temperatures of the perfluoroalkylsulfonic acid polymers, solid polymer electrolyte membranes that have aromatic polymers used in engineering plastics have been developed.
For example, U.S. Pat. No. 5,403,675 (Patent Document 1) discloses solid polymer electrolytes comprising a rigid-rod sulfonated polyphenylene. The polymer is obtained by synthesizing a polymer of an aromatic compound composed of phenylene units, and then introducing a sulfonic acid group by reaction with a sulfonating agent. The electrolyte membranes of this polymer have a thermal deformation temperature of 180° C. or above and are excellent in creeping resistance at high temperatures. However, they require a very high temperature when assembled with electrodes by hot pressing. Long heating at high temperatures induces elimination reaction of the sulfonic acid groups, crosslinking among the sulfonic acid groups, and degradation of electrode layers.
Further, they are insufficient in properties for use as proton conductive membranes in direct methanol fuel cells.
Patent Document 1: U.S. Pat. No. 5,403,675